1. Field of Invention
This present invention generally relates to cerebrospinal fluid shunts and, more particular, to a device and method for testing for the presence, absence and/or rate of flow in the shunt tubing implanted under the skin.
2. Description of Related Art
The Surgical Management of Hydrocephalus
Hydrocephalus is a condition of abnormal cerebrospinal fluid (CSF) homeostasis, resulting in an accumulation of CSF in the brain ventricles. Approximately 69,000 people are diagnosed with hydrocephalus each year in the United States, most commonly as a congenital condition, making it one of the most common birth defects [1]. Untreated hydrocephalus leads to progressive neurological dysfunction and death.
The most commonly used treatment for hydrocephalus is diversion of CSF from the ventricles to the peritoneal cavity by means of a permanent prosthetic shunt. A CSF shunt is comprised of a valve connected to a tube. The proximal end of the tube is surgically inserted into the ventricle of the brain, and runs subcutaneously through the body into the abdominal cavity (FIG. 4). There are approximately 300,000 shunted hydrocephalus patients in the US. 41,000 [3] shunt procedures are performed each year, approximately 12,000 of which are new shunt placements [4].
Improved materials, diagnostic, and treatment technologies, have improved shunt therapy since the 1970s [14]. However, shunt failure is still almost inevitable during a patient's life. The one-year failure rate of ventriculoperitoneal shunts has been estimated to be approximately 40% [15, 16], and the mean period to failure of an implanted shunt is typically only 5-10 years [17]. Obstruction of the ventricular catheter, usually from tissue ingrowth or clots, is overwhelmingly the greatest cause of shunt failure [4, 15, 18-20]. Shunt failure can rapidly progress to life-threatening elevation in intracranial pressure, so revision surgery, and re-placement of the blocked ventricular catheter is indicated. More than half of all shunt procedures in the United States are revisions [1, 4].
However, since catheter replacement surgery carries risks of life-threatening complications such as infection or embolism, a need for shunt revision needs to be reasonably established. The problem is that the usual clinical manifestations of shunt failure, such as headaches, vomiting, or loss of vision, are non specific and are difficult to differentiate from common, less serious illnesses, particularly in pediatric patients. This leads to two extremes of management: patient families who present persistently at emergency rooms for every headache or flu symptom, and patient families who dangerously dismiss symptoms of a shunt blockage as a common ailment. A study at the Children's Hospital of Philadelphia (CHOP) [4] indicates that three false alarms are seen for every true shunt malfunction. There is a need for objective methods to evaluate suspected shunt obstruction.
New Methods for Diagnosing Shunt Obstruction are Needed
An unacceptably high number of hydrocephalic children still die as a result of shunt malfunction, primarily because of a failure to identify shunt blockage at an early stage [14]. The early diagnosis of shunt obstruction is complex and difficult. While a number of shunt flow detection methods are available, none are diagnostic when used alone or are without complication, and there is little standardization to guide physicians in their interpretation (Table 1). Physical examination of the patient, including pumping of the shunt reservoir, is unreliable [21]. Measuring CSF pressure by “shunt tap” is invasive, painful, and can be misleading [22, 23]. CT (computed tomography) and MR (magnetic resonance), either alone, or in combination with plain radiographs, remain the gold standards for diagnosis of shunt malfunction [4, 9]. However, these imaging techniques are static, and so must be performed multiple times to detect ventricular enlargement. This results in repeated radiological exposures of patients (often children), a safety concern for pediatric neurosurgeons [40]. Furthermore, the reliability of these techniques for detecting CSF accumulation has been questioned [9, 24]. For a while, radionuclide markers were widely used to derive truly dynamic information about CSF flow in the brain and in shunts [25-27]. However, their promise was never wholly realized, and they are not routinely utilized in most clinical settings. Because of the expense and technical complexity of advanced imaging techniques, they cannot be used to investigate every headache.
TABLE 1Performance of commonly used diagnostic procedures for suspected CSF shunt obstructionSensitivitySpecificityDiagnostic(Detecting No(DetectingProcedureFlow)Flow)FeaturesStatic Imaging ProceduresCT Scan [36]68%90%Expensive, time-consuming, radiation dose. Shuntmalfunction must have gone on long enough for the scan todetect visible changes, i.e. ventricle enlargement. Risingconcern about radiation.X-ray Series [36]27%99%Expensive and time-consuming. As with CT, the shunt musthave malfunctioned long enough for visible changes to bedetected.Dynamic Flow MeasurementsShunt Tap [37]79%56%Method is painful, risks infection and can be inconclusive ifblockage is upstream of the tap area.Radio Isotope80%53%Requires an invasive shunt tap and 24 hours lead time for[38]isotope. This method is considerably more involved thaneither the CT or MRI.
The current, non-invasive imaging procedures have relatively low sensitivity and better specificity—making them reasonable rule-in tests but poor rule-out tests. The invasive procedures are somewhat better rule-out tests, but are painful and present an infection risk. Furthermore, children are often sent to CT scans, the most commonly used procedures, when they present to the emergency room (ER) and such repeat exposure to radiation may be harmful.
Therefore, there remains a simple and reliable method for determining CSF shunt flow rates that can be interpreted by neurosurgeons and non-neurosurgeons with equal confidence.
All references cited herein are incorporated herein by reference in their entireties.